The invention relates to a fuel system for a gas turbine plant, which has a heat exchanger for cooling and/or a heat exchanger for heating gaseous fuel, the cooling or heating medium being water which flows in a water circuit. The invention relates, in particular, to a gas/liquid separation apparatus in this water circuit, and to a process for separating gas from such water. The separation apparatus serves to separate gas from the water and to detect the gases separated from the water, and thus to increase the operational reliability of the system. Denoted below as fuel is a gaseous fuel such as, for example, CH4, C2H6, CO, H2 or a gas mixture.
Gaseous fuel which is intended for a gas turbine plant is generally led, coming from a natural gas feed line, firstly through a fuel system in which the parameters, required for the further combustion of the fuel, are controlled.
For example, temporal pressure fluctuations of the fuel occur in the natural gas feed line. In order to equalize these fluctuations, the pressure in the fuel system is appropriately raised or reduced. After a pressure reduction, the fuel can have, for example, a temperature which could cause icing of valves or damage to other subassemblies of the plant. In order to prevent this, the fuel is, for example, already heated before the pressure reduction in a heat exchanger, a so-called dew-point heater. Hot water, for example, is used as heating medium for such dew-point heaters. Furthermore, such a fuel system can be used to enhance or optimize the efficiency of the gas turbine plant. The fuel is preheated for this purpose in a heat exchanger to a prescribed temperature.
A system for heating the fuel in order to enhance or optimize the efficiency of the gas turbine plant is disclosed, for example, in EP 0 918 151, which is hereby incorporated by reference. There, the fuel is guided through the tubes of a heat exchanger around which water flows as a heating medium which flows in a water circuit and has been preheated in a waste-heat recuperator of a gas turbine plant. In particular, the pressure of the fuel is higher than that of the water, in this case.
In particular, the pressure of the fuel is higher than that of the water, in this case.
In order to control a gas compressor for example in a load range below 80%, the fraction of the mass flow not going to the gas turbine plant is recirculated and thereby fed once again to the compressor inlet. Connected in the recirculation line is a heat exchanger in which the fuel is cooled by means of water. Heating of the compressor is thereby avoided in the recirculation operation.
The fuel is led for this purpose through the tubes of the heat exchanger, about which cooling water flows. The cooling water is pumped by means of a delivery pump through a cooling-water circuit which essentially comprises the delivery pump, the heat exchanger, a water-to-water cooler and a gas/liquid separation apparatus. The pressure of the fuel is significantly higher than that of the cooling water in the heat exchanger for cooling the fuel.
Since in both above described heat exchangers, for cooling and for heating the fuel by means of water, the pressure of the fuel is higher than that of the water, there is the risk of a leak occurring, in which case fuel passes into the water. This results in a two-phase flow in the water circuit, which constitutes a safety risk for operating the plant. For example, there is the risk of ignition or explosion in the case of mixing with air at the suction connection of the delivery pump. Furthermore, running dry or cavitation of the delivery pump can occur. Consequently, it .is very important to detect gas leaks at an early stage in order to avoid sizable damage in the system.
In order to limit the level of risk, a gas/liquid separation apparatus, called a separation apparatus here, for short, is connected in the water circuit downstream of the heat exchanger. It serves to separate gases contained in the water, and to detect the gases separated from the water. These are principally methane, hydrogen and further combustible gases. For this purpose, the gases are fed via a gas extraction line to a gas detector, for example a mass spectrometer. In the case of a measurable gas concentration, an appropriate warning is then released, so that necessary countermeasures can be undertaken.
Conventional separation apparatuses which are currently being used have no defined water level as a free surface in the separation apparatus itself, at which the gas bubbles entrained in the water can escape and pass into a gas extraction line. Consequently, the separation and detection of the gases is not sufficiently ensured, with the result that it is not possible to detect a leak reliably in good time.
In view of the above-named prior art, it is the object of the invention to create a gas/liquid separation apparatus for a water circuit of a fuel system for a gas turbine plant, and a process for separating gas from the water, by means of which the separation of gaseous fuel from the water is improved so that the reliability of the gas detection is raised and, consequently, the level of risk for the operation of the water circuit of a fuel system is reduced.
This object is achieved in accordance with the invention by means of a gas/liquid separation apparatus in a water circuit of a fuel system for a gas turbine plant which has a container with a water inlet and water outlet, and a gas extraction line which leads to a gas detector. According to the invention, the separation apparatus is partitioned by a separating wall in the interior of its container into a water entry chamber and a water exit chamber which are hydraulically interconnected at least in the upper region of the container, thus producing in the water entry chamber a columetric flow flowing upward from the water inlet. This columetric flow then flows from the water entry chamber into the water exit chamber and from there into the water outlet. Furthermore, particularly in the region of it upper termination, the container has a line for feeding into the container a gas which fills a space above the water, as a result of which the water in the container has a free surface.
The partitioning of the container volume into a water entry chamber and a water exit chamber is intended to form a calming section in the water entry chamber at which gas can be separated from the water, and a water outlet in the water exit chamber, which is separated from the calming section. In this case, the free surface is defined by the pressure in the gas-filled space. The hydraulic connection in the upper region of the two water chambers produces in the water entry chamber an upwardly flowing volumetric flow with the aid of which gas bubbles disperse in water rise. The calming section also effects an upwardly directed, non-turbulent flow with a homogeneous velocity profile. This is achieved in conjunction with a suitable duct guidance and low Reynolds numbers, re  less than 700 for water, for example. The gas bubbles can escape at the free and calm surface of the calming section without disturbance from the water into the gas-filled space situated thereabove. In the process, the gas fed into this space differs from the gases from the water. The water exit chamber, which is separate from the water entry chamber, permits an irrotational water outlet, as a result of which as few gas bubbles as possible are entrained into the water outlet.
In a preferred embodiment, the two water chambers are hydraulically connected in their upper region as follows.
The partition which separates the two chambers from one another is tightly connected to the base of the container. There is a free space between the upper edge of the partition and the upper termination of the container. The water flowing upward in the water entry chamber flows over the edge of the partition in the manner of an overflow weir into the water exit chamber. Because of the large width of the overflow weir, slight cross flows prevail in the calming section, with the result that a small number of gas bubbles are entrained by such flows into the water exit chamber. The majority of the gas bubbles which have risen escape instead of this without disturbance into the gas-filled space.
In a variant, the hydraulic connection between the two water chambers comprises one or more openings in the upper region of the partition.
In an advantageous embodiment of the invention, the partition of the gas/liquid separation apparatus has in the region of the base of the container a plurality of openings through which water can flow from the water entry chamber into the water exit chamber. These openings split up the water flow in the water entry chamber into a smaller downwardly flowing volumetric flow and a larger upwardly flowing volumetric flow. The openings ensure an additional hydraulic connection between the two water chambers, and permit the control of the division of flow and of the water level in the water entry chamber.
The additional hydraulic connection between the two water chambers also ensures hydrostatic pressure equalization even in the case of standstill.
In a further design, the separation apparatus has a pressure control valve in the line for feeding the gas into the container, and a level-measuring unit in the water exit chamber. These serve the purpose of controlling the water level in the water exit chamber by virtue of the fact that the gas pressure in the gas-filled space is controlled in accordance with a desired value for the water level, the water level in the water exit chamber being kept at a level below the upper edge of the partition.
For the case of a sudden relatively large gas leak, for example in the case of tube fractures, the container has at its upper termination a safety valve via which gas can escape into the outside, with the result that the container pressure remains within a permissible region.
In a preferred design of the separation apparatus, the partition is angled in a V shape in its horizontal cross section between the two chambers. The smaller, inner angle of the V-shaped partition faces the water exit chamber, the inner side of the V-shaped partition forming the water exit chamber together with the container inner wall. The larger, outer angle of the V-shaped partition faces the water entry chamber, the outer side of the V-shaped partition surrounding the water entry chamber together with the container inner wall. Given appropriate installation, the water entry chamber therefore has a larger volume than the water exit chamber. Furthermore, the V shape of the partition lends an increased stiffness, with the result that the flow loadings such as, for example, buffeting or so-called xe2x80x9cwater hammeringxe2x80x9d can be more effectively resisted. In addition, the V shape of the partition permits an enlargement of the overflow width of the overflow weir for a given container cross section.
In a variant of this design, the partition has a zig zag weir line, as a result of which the overflow width of the weir is additionally enlarged.
In a further design, the partition is shaped in the form of a cylinder envelope.
In a preferred design, the water exit chamber is shaped so as to achieve an irrotational flow in the direction of the water outlet tube. In one design, the water exit chamber has built-in components for calming the outlet. In a further design, the outlet is implemented by a single-limbed or two-limbed U tube, one limb of the U tube projecting into the water exit chamber, and the other being located outside the container.
The container also has in the region of the container base a drainage nozzle for the purpose of emptying at standstill.
In a process in accordance with the invention, for the purpose of separating gas from the water of a water circuit in the fuel system described at the beginning the water is firstly led into a container where it flows upward at a uniform speed in a water entrance chamber and entrains gas bubbles in so doing. At the free surface of a calming section, in the water entry chamber the gas bubbles escape from the water into a gas-filled space above the water surface, the free surface being defined by the gas pressure in this space. The water flows via a hydraulic connection into a water exit chamber, which is separated from the water entry chamber by a partition, and flows irrotationally in the water exit chamber into a water outlet.
An undisturbed rise of the gas bubbles in the water entry chamber toward the water surface is effected by as large as possible a volume of the water entry chamber and the rate of rise of the water which is thereby as low as possible. The gas bubbles escape into the gas-filled space at the water surface of the calming section. The cross flows in the calming section are reduced by means of as wide as possible an overflow weir, as a result of which the surface is as calm as possible, and the gas bubbles can more effectively escape from the water. This prevents gas bubbles from being entrained into the water exit chamber and not being detected.
The height of the water level of the weir in the water entry chamber is a function of the incoming volumetric flow of the water and of the overflow width of the overflow weir. The level in the water exit chamber is controlled by the pressure of the gas in the gas-filled space, the water level in the water exit chamber being kept below the upper edge of the partition. For this purpose, a valve for the gas inflow and the gas pressure in the gas-filled space is opened and closed in accordance with a desired value for the water level in the water exit chamber. A change in the height of this water level effects a change in the driving pressure gradient via the openings in the region of the container base, and simultaneously also in the volumetric flow distribution. The pressure difference active for the openings is limited in this case by the sum of the overflow height of the weir in the water entry chamber and the height of the level of the water exit chamber.
The size of the openings in the partition is determined in accordance with the desired volumetric flow distribution between the upwardly and downwardly flowing water flow, and in accordance with the desired speeds for the two water flows. They are dimensioned, in particular, such that the speed of the water flow resulting downward does not exceed the rate of rise of the smallest gas bubbles to be separated. The critical diameter of these gas bubbles in the case of a selected downwardly directed water speed of 0.01 m/s is 130 xcexcm. As a result, with this design gas bubbles which are larger than 130 xcexcm pass upward to the water surface.